THINKING BIG: Before finding TV success as the egotistical police chief on "Hill Street Blues" and commanding general on "Major Dad," Jon Cypher was a busy New York theatre actor in roles as varied as the villainous Dr. Carrasco in Man of La Mancha and Katharine Hepburn's father(!) in Coco. It took Big to get Cypher back to Broadway, where he's having a terrific time playing the toy company mogul who hires Josh Baskin, a 12-year-old boy trapped in an adult's body.

THINKING BIG: Before finding TV success as the egotistical police chief on "Hill Street Blues" and commanding general on "Major Dad," Jon Cypher was a busy New York theatre actor in roles as varied as the villainous Dr. Carrasco in Man of La Mancha and Katharine Hepburn's father(!) in Coco. It took Big to get Cypher back to Broadway, where he's having a terrific time playing the toy company mogul who hires Josh Baskin, a 12-year-old boy trapped in an adult's body.

"This show is positive; it's romantic, it's funny, and there's a certain magic about it," says the Cypher, whose real-life speaking voice is much smoother and more youthful than the self-involved characters he plays on TV. "We had a matinee audience today with a lot of teen-agers, and they went crazy."

Settling in for a year-long run, Cypher finds his old Upper West Side neighborhood remarkably different than when he left the city 25 years ago. "Columbus Avenue used to be a pit," he says. "Now there are zillions of great restaurants and health food stores. It's just a much more delightful city to be in now."

Cypher reports that his wife, Dr. Carol Rosin, president of the Wash- ington, D.C.-based Institute for Security and Cooperation in Outer Space, is a Broadway fan as well as a crusader against space-based weapons. "When she's in town, she stands in the back and sees every show. She doesn't like Hollywood, but she loves the theatre."